The present invention relates to communications among a group of moving participants and, more particularly, to a method for selecting, in real time, which of a number of communication channels is best for broadcast communications among the participants.
The Figure illustrates a scenario to which the present invention is applicable: naval maneuvers. A group of participants: ships 10, 12, 14, 16 and 18 and an aircraft 20, are engaged in war games. At various times during the war games, it is necessary for one of the participants to broadcast a message that will be received essentially simultaneously by all participants. For example, ship 10 may need to notify all the other participants that it has simulated the firing of a missile 26.
The participants in military maneuvers typically have several communication channels at their disposal, each channel having its own characteristics. As an illustrative example, the participants in the naval maneuvers of the Figure have at their disposal the following channels:
______________________________________ typical channel geometry range latencyate ______________________________________ S-band line-of-sight 40 nautical miles high 200 .mu.s HF 300 nautical miles very low 18 msec satellite indirect global 0.2 secery high ______________________________________
The S-band channel is fast but is limited to line of sight communications. The "40 nautical miles" range is for communications between aircraft 20 and the other participants; the effective range between ships is limited to 15 nautical miles. At longer ranges, the much slower adaptive HF channel is used; and at still longer ranges, a satellite system, based on Earth satellites such as satellite 22, is used.
Because the three channels have different latencies, the signal from ship 10, that missile 26 has been launched, arrives at the other participants at different times. If all the participants had fixed locations, permanent broadcast communication channels could be selected according to the fixed distances between the participants, and the latencies of the channels could be built into the communications protocol. This cannot be done in this case because the participants are moving. For example, ship 18 is shown in the Figure out of S-band range of ship 12 but about to move within S-band range of ship 12. At the time of the Figure, ships 12 and 18 communicate using the adaptive HF channel, but they shortly will switch to the S-band channel.
There are two inferior solutions to this problem. One is for all participants to broadcast over all available channels, including in each signal an indication of the channel used, so that each recipient of a signal can compensate for the latency of whichever channel succeeds in relaying the signal to the recipient. This wastes bandwidth. The other solution is for all participants to use the channel with the longest range, which typically, as in this example, is also the channel with the longest latency, so that all signals have the same latency. The channel with the longest range typically, as in this example, also is the most expensive channel. Ships 10 and 12, which are in line-of-sight of each other, could communicate at less cost via the S-band channel than via satellite 22.
Tzidon et al., in U.S. Pat. No. 5,396,644, describe a communications method intended to make most efficient use of a short range communications system. According to this method, each participant keeps track of which other participants are within communication range, and available bandwidth is allocated so that no attempt is made by participants who are out of range of each other to communicate with each other. This method could be used, in the above example, to allocate S-band bandwidth most efficiently, but it is inherently limited to one communications channel, and cannot be used to allocate multiple communications channels among all participants.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method by which moving participants could select among communications channels based on their instantaneous locations.